Wafers, in the microelectronic industry, are often coated with varying metals to facilitate such things as component interconnection with the wafer. Coating wafers with different metals is often accomplished with such deposition techniques as electron beam evaporation or sputter deposition. However, for depositing relatively thick films onto wafers, electroplating has become the most commonly used technology. When plating wafers, often the metal used in the plating is a precious metal such as gold or platinum. Obviously, with precious metal platings it is desirable to reduce the amount of plating material lot to waste. Both electron beam evaporation and sputter deposition create more waste when depositing thick films than does electroplating. Consequently, when electroplating can be used, it is the most cost effective method of metal film deposition.
When used on wafers, electroplating is not without disadvantages. Often, only one side of a wafer needs to be coated. With other techniques, such as electron beam evaporation and sputter deposition, a one-sided coating is easy to obtain. However, with electroplating all the surfaces that are submersed into the electroplating solution may incur some degree of plating. To limit metal deposition on unwanted areas of wafers, wafers must be masked with a dielectric material such as a mylar film. The application and removal of masking material, before and after electroplating, reduces the efficiency of the overall process.
Another disadvantage of electroplating wafers is the turbulent environment of an electroplating tank. Electroplating solution is often circulated during the plating process to assure uniformity in the deposited materials. Wafers are planar and are also often brittle. The planar shape of the wafer is easily influenced by the flow of the electroplating solution. Consequently, wafers may break or crack during the electroplating process. Some cracks may be obvious and the wafer easily discarded, however some cracks may be microscopic and may cause failure of the wafer only after an extended period of time or repeated thermocycling.
Yet another disadvantage of electroplating wafers is controlling the rate of metal deposition. The deposition rate of electroplating depends on many factors such as the density of the metal ions in the electroplating fluid, the electrical coupling of the wafer to a cathode source, the frequency of the current passing through the wafer, and the waveform of the current. It is only by controlling these factors that an accurate plating thickness can be manufactured without the need for repeated measurements of the plating thickness during the electroplating process.
It is therefore a primary objective of the present invention to set forth an apparatus and method for electroplating wafers wherein only one side of a wafer is electroplated without the need of masking film, the wafer is not subject to fluid turbulence during the electroplating procedure and the operating parameters of the electroplating process can be maintained at predetermined values.